This invention relates to background lighting apparatus and more particularly to thin background light sources for liquid crystal panel displays, such as, employed in computers, word processors and LCD television sets and the like.
In Japanese Laid Open No. 63-62105, there is described a background light system in which the irregular reflecting surface is provided with a pattern of milky white type, dispersion points on a transparent plate supported in the housing unit of the liquid crystal display. FIG. 1 discloses the irregular reflecting layer of the type disclosed in Japanese Laid Open No. 63-62105 comprising two or more transparent plates 1A and 1B, each having a pattern of milky white type of light diffusion pattern comprising a multiplicity of dots 50 formed on bottom reflecting surfaces 10 of plates 1A, 1B. The backlighting apparatus disclosed includes opposed light sources 6 for providing light to enter opposite end surfaces of dual plates 1A, 1B. Diffusion plate 4A permits the transmission of light beams from plates 1A, 1B as well as brings about diffusion of light beams that are not at the necessary critical angle for exiting the apparatus. Total reflecting plate 5A is of the white type for the dispersion and reflection of light back into plates 1A, 1B. This type of dispersion pattern and dispersion/reflection plate approach improves the quantity of light reflections, diffusion and dispersion but it does so at the expense of providing two or more plates 1 in order to increase the output efficiency of the background lighting apparatus. Furthermore, the luminous efficiency of this type of reflecting surface is low when employed in the rear portion of the liquid crystal display housing unit. In order to increase the brightness, a plurality of superimposed transparent plates 1A, 1B are utilized each halving a light diffusion pattern for diffusing the light beam. However, the thickness of the display becomes large. Further, the dispersion and diffusion of light and resulting brightness in the reflecting surface or multiple surfaces of the transparent plates is not achieved resulting in unevenness or nonuniformity in brightness in back lighting and, correspondingly, in the displayed image.
In general, a background lighting apparatus for a liquid crystal display is required to have uniform brightness across its output facet or face. In this type of apparatus, wherein light enters from an edge or end face of the transparent plate of the apparatus and is guided internally in the transparent plate to form a plate type lighting apparatus, such as in the case of this invention, the total quantity of light output can be increased by increasing the quantity of irregular reflection occurring relative to the irregular reflecting surface or portion provided relative to the transparent plate thereby resulting a brighter background light. However, ais the quantity of irregular reflection is increased, the brightness level quickly decreases with distance from the light source so that nonuniformity in the overall brightness of the light output from the lighting apparatus increases. In order to counteract this phenomenon, a pattern comprising an irregular reflecting surface or portion and a totally reflecting portion are arranged in a manner to achieve uniform light output with a high level of brightness. As previously indicated, this has been accomplished in the past by a patterned surface on a transparent plate as described in Japanese Laid Open No. 63-62105. However, it is difficult to increase the area ratio of the irregular reflecting portion over the regular reflecting portion because, in doing so, the overall light output efficiency tends to decrease.
FIG. 2 illustrates another conventional background lighting apparatus principally consisting of light sources and a light guiding system. Light sources 6 are provided at opposite ends of transparent plate 1 adjacent to its edge surfaces 4. The design of this conventional apparatus has low luminance and is insufficient as a backlighting apparatus for a liquid crystal display structure, primarily in the case of a color display, because the color illumination is not uniform across the display. On manner of solving this problem is simply to increase the luminance of the light sources. However, this accompanied by a corresponding increase in exothermic generation, i.e., heat generation, which greatly effects the quality of the color image that is produced. Moreover, there is the situation that the temperature rise is greatest in the vicinity of the light sources 6 at opposite ends of plate 1 so that the heat generated is greatest at the vicinity of plate edges 4 and drops exponentially from these edges toward the center of transparent plate 1. As a result, the view of an image is not uniform across the display. This is because the threshold voltage for operation of the plurality liquid crystal elements of the liquid crystal display are affected by the temperature distribution generated by the background lighting apparatus. As a result, the threshold voltage of operation of the liquid crystal elements will shift and be different in regions of the display where the temperatures are different in the display structure. Thus, if the applied voltage level for the liquid crystal elements changes and is different in different locations of the display matrix, a significant problem is created in overall control of the display. This, in particular, has an affect on display of gray scale since a shift in light/voltage characteristics due to temperature differences will also shift the limited range of applicable gradation, particularly in the case of color displays.
The backlighting in liquid crystal displays has also been provided on the read surface or side of the liquid crystal structure. In order to provide uniform illumination with evenly distributed light intensity and luminescence, high luminance is required. In order to meet the requirements of uniformity in illumination, it is also necessary that a uniformity in temperature distribution be maintained. This is highly important from a standpoint of operational characteristics of the display since the transmissivity of the liquid crystal material is a function of temperature and can vary in a significant manner depending upon ambient temperature operating conditions. Therefore, if the surface temperature at the back surface of the display structure differs at different locations of the display, there is a serious problem in terms of nonuniformity in luminance and color output quality in operation of the display. Thus, the uniformity in temperature distribution across the display structure and, correspondingly, luminance uniformity are critical to producing a commercially acceptable liquid crystal display system.
It is, therefore, an object of this invention to provide a background lighting apparatus for liquid crystal display panels or structures, or other such displays requiring background lighting, having high luminous efficiency and uniformity in luminance with a uniform temperature distribution across the display structure.